It is known in a converting device in which a process quantity, such as pressure or the like, is converted into an electric signal and the electric signal is subsequently transmitted, to use DC current signals of 4-20 mA or 10-50 mA. These signal levels are widely employed because the converted signal is less affected by induced noise voltages and can be transmitted over a long distance. Generally, the receiving measurement instrument for receiving the above-mentioned current signal is arranged such that the current signal is converted into a DC voltage signal of 1-5 volts by a fixed resistor connected in series with the signal transmission line and then the voltage signal is received. The power consumption is normally several hundred mW or more in such a converting device of a current signal transmission system.
There have been many cases where it is required to locate the converting devices in distant places as the area of process control become widespread. In such cases, if the converting device is driven by a commercial power source, it becomes necessary to construct a power line from the commercial power source to the distant areas. Alternately, if the converting device is arranged to be driven by a battery, the power consumption is large and battery maintenance work, such as charging, etc., becomes quite troublesome. These problems are common particularly in converting devices for detecting mechanical displacements and converting the detected values into electric signals representing flow rate, pressure, and other variables widely used for process control.